Considerable efforts have been put into elucidating the biochemical mechanisms involved in the detection, transduction and transmission of hot and cold sensations in neuronal tissues. Thermal stimuli activate specialized receptors located on sensory neurons, such as those deriving from the dorsal root ganglion (DRG) and the trigeminal ganglion (TG). When these stimuli are in the noxious range (i.e, very hot or cold), they activate a certain subset of thermal receptors on a sub-population of sensory neurons called nociceptors (pain-sensing neurons). Upon activation, the thermal receptors (e.g., ion channels) transduce the noxious stimulus into an electrical signal that is propagated along the sensory neuron to the spinal cord, where it is relayed to the brain, ultimately leading to the perception of pain. Accordingly, these thermal receptors represent highly promising targets for developing drugs for the treatment of various painful conditions.
Several temperature-activated receptors have been implicated in sensing heat. TRPV1 (VR1: a capsaicin- and heat-activated channel) is activated near 43° C., a temperature most mammals perceive as noxious. Other TRPV channels with greater than 40% amino acid level identity to TRPV1 also have been cloned and characterized as thermosensors. These channels are activated at various heat thresholds, ranging from 39° C. (warm) for TRPV3 to 55° C. (high-threshold noxious heat) for TRPV2/VRL1 (See Story et al., Cell, 2003, 112:819-829, and references therein). In contrast, TRPV4 is constitutively opened at room temperature being activated at temperatures greater than approximately 27° C. (Güler et al., J. Neurosci. 2002). These temperature-activated receptors belong to the transient receptor potential (TRP) family of non-selective cation channels, which in C. elegans and D. melanogaster are involved in mechano- and osmoregulation. TRP channels are divided into three subfamilies designated TRPC (canonical or capacitive subfamily), TRPV (vanilloid subfamily), and TRPM (melanostatin subfamily). All have six putative transmembrane domains with a proposed pore region between transmembrane domains five and six. TRP channels are thought to have cytoplasmic N- and C termini (See Story et al., supra, and references therein).
More recently, proteins have been discovered that fall within the TRP family of proteins and modulate responses to cold stimuli. A rat CMR1 protein (for “cold- and menthol-sensitive receptor”; McKemy, D. D., et al., Nature, 416:52-58, 2002) and a mouse TRPM8 protein (for “transient receptor potential channel, melanostatin subfamily, type 8”; Peier, A. M. et al., Cell 108:705-715, 2002) appear to function as excitatory ion channels that are activated upon exposure to relatively low temperatures. The threshold of TRPM8 activation is approximately about 23° C. The rat CMR1 and mouse TRPM8 are also sensitive to compounds that provoke cold sensations, such as menthol and icilin. Interestingly, the rat CMR1 and mouse TRPM8 share over 90% sequence identity over the entire length of their amino acid sequences.
There is a need to identify additional thermal receptors, as they are potential targets for the treatment of pain. There is also a need to identify thermal receptors in different species, as they can be used as model systems to investigate the effects of test compounds. Particularly, there is a need for systems that can be used to test compounds that potentially increase or decrease the activity of a thermal receptor responding to cold stimuli. Identification and testing of such compounds would enable the treatment of various disorders associated with chronic pain or for uses in other conditions in which tissue cooling is desirable.